The present application is related to the co-pending, commonly assigned application entitled, xe2x80x9cMethod for All Order Dispersion Compensationxe2x80x9d, Ser. No. 10/037,024 filed co-currently with the present application, which is hereby incorporated by reference.
The present invention relates to a method and system for polarization mode dispersion compensation of optical signals. In particular, the present invention relates to a method for polarization mode dispersion compensation using at least two linearly chirped Bragg gratings to selectively tune the reflection points of two polarization resolved signals, creating a variable polarization dependent delay.
Present day telecommunication systems require that optical signals be conveyed over very long distances. In an optical communications signal, data are sent in a series of optical pulses. Real signal pulses are composed of a distribution of wavelengths and polarizations, each of which travels at its own characteristic velocity. This variation in velocity leads to pulse spreading and thus signal degradation. Degradation due to the wavelength dependence of the velocity is known as chromatic dispersion, while degradation due to the polarization dependence is known as polarization mode dispersion.
Mathematically, the speed of light v in a waveguide is given by                     v        =                  c          n                                    (        1        )            
where c is the velocity of light in free space and n is the effective index of refraction in the waveguide. Normally, the effective index, n, of the optical mode is dependent upon the wavelength. Thus components of light having different wavelengths will travel at different speeds. In addition to being dependent upon wavelength, the effective index in a waveguide may also be dependent upon the polarization of the optical signal. Even in xe2x80x9csingle-modexe2x80x9d fiber, two orthogonal polarizations are supported, and, in the presence of birefringence, the polarizations travel at different speeds. Birefringence in the fiber may arise from a variety of sources including both manufacturing variations and time-dependent environmental factors. The speed difference results in a polarization-dependent travel time or xe2x80x9cdifferential group delayxe2x80x9d (DGD) between the 2 different polarization modes within the birefringent fiber. In real systems, the degree of birefringence, and the orientation of the birefringent axes, varies from place to place along the fiber. This results in a more complex effect on the optical signal, which is characterized by the concept of xe2x80x9cprincipal states of polarizationxe2x80x9d or PSPs. PSPs are defined as the two polarization states that experience the maximum relative DGD, and they uniquely characterize the instantaneous state of the system
Polarization mode dispersion (PMD) is the distortion arising from the statistical sum of the different group velocities of the two components of polarization as the signal propagates through the different sections of the optical communications system. PMD includes first order PMD and higher order PMD and is non-deterministic. First order PMD is the differential polarization group delay at a given wavelength. The instantaneous value for a long fiber can vary over both long time intervals (due to slow variations, such as temperature drift) and short time intervals (due to fast variations, such as mechanical vibration induced polarization fluctuations). The coefficient describing the mean value of first order PMD can vary from  greater than 2 ps/km1/2 for relatively poor PMD performance fiber to  less than 0.1 ps/km1/2 for relatively good PMD performance fiber.
Second order PMD arises from two sources: i.) a first order PMD that varies with wavelength; ii.) a change of the system PSP (principal state of polarization) orientation with wavelength, which results in a variation of PMD with wavelength. Second order PMD results in a wavelength dependent group delay, which is equivalent in effect to variable chromatic dispersion, and, can have either a negative or positive sign. The speed of fluctuation is similar to that of first order PMD.
Dispersion imposes serious limitations on transmission bandwidth, especially across long distances, such as in transoceanic routes. Dispersion issues become much more important at higher bit rates, where the separation between the optical pulses is less and where shorter pulses result in a wider signal spectral bandwidth, exacerbating chromatic and second order PMD effects. At bit rates greater than or equal to 40 Gb/s, even for good fiber ( less than 0.1 ps/km1/2 PMD) long length links are deemed to require PMD compensation. PMD can become an inhibiting factor either limiting overall system length or increasing system costs due to the need for additional optical-to-electrical-to-optical signal conversion sites to permit electrical signal regeneration.
One approach to compensation for first order PMD is to introduce a DGD of equal magnitude and opposite sign to the first order PMD in the system. In general, time delays in an optical system can be described in terms of optical path length (OPL) defined by
xcex94=nLxe2x80x83xe2x80x83(2) 
where xcex94 is the OPL, L is the physical length of the medium, and n is the index of refraction of the material.
As may be appreciated from equation (2) above, the OPL of an optical waveguide may be lengthened by increasing the index of refraction of the medium or by increasing the physical length of the waveguide. Similarly, the OPL of an optical waveguide may be shortened by decreasing the index of refraction or by decreasing the physical length of the waveguide. Thus, to generate a DGD for PMD compensation, the two orthogonal PSPs of the signal can be sent down two separate paths with different OPLs. If the delayed polarization of the signal is sent down a path with a shorter path length than the leading polarization of the signal, the amount of differential group delay between the two polarizations will be reduced.
A variety of alternatives have been presented to attempt to compensate for first order PMD effects. One proposed system includes a polarization controller and a length of high birefringence polarization maintaining (PM) fiber. A photodetector samples the output signal and attempts to drive the controller using control loop techniques. A long coil of PM fiber (e.g., 50 meters) is necessary to achieve adequately large DGD for dispersion compensation. More importantly, the amount of PMD correction is fixed because of the fixed DGD of the PM fiber, limiting the adaptability and applicability of the system.
Another proposed system attempts to address the problem of adaptability by employing a movable prism element, which generates a variable DGD by varying the distance traveled by one polarization. There are a number of disadvantages to this scheme. For example, optical path losses must be very closely balanced to prevent polarization-dependent loss (PDL). In addition, the overall speed of the variable delay element will be slow due to the mechanical movement of the optics. Furthermore, since the variable delay is created outside of fiber there may be issues of cost and stability due to the complexity associated with the required active alignment of the optical beam.
Another proposed approach to a variable DGD element consists of a single non-linearly chirped grating in a PM fiber. Chirped gratings are gratings in which the spacing of the grating elements varies with position along the grating, so that the effective position at which a signal is reflected depends on its wavelength. In this case, the application of axial strain to the fiber changes the reflection location of each polarization at a different rate, thus changing the delay between them. However, such a design can only achieve a limited range of delays because the differential delay is proportional to the small birefringence of the fiber, and is limited by the small range of strain that the fiber can withstand before breaking. Additionally, this approach induces a varying chromatic dispersion that must be separately compensated
Yet another proposed approach to generating a differential group delay (DGD) consists of a polarization beam splitter coupled to a pair of single-mode (SM) optical fibers each having a linearly chirped Bragg reflection grating and a controllable extension means for differentially axially straining the fibers. This dual grating approach has the advantage that the two polarization components experience the same chirp when reflected, thereby experiencing matched chromatic dispersion so that polarization-dependent chromatic dispersion is not introduced. However, such a system does not account for polarization fading effects which will require dynamic polarization control in each of the SM fiber grating paths to assure that all light returns properly through the polarization splitter; this will substantially add to the complexity and cost of the system. Furthermore, such a system does not address the difficulty of balancing the two legs during manufacturing, or of properly biasing the system when the compensator is first turned on.
The need remains for a reliable, wide-dynamic range, dynamically tunable PMD system.
The present invention relates to a dynamically tunable polarization dispersion compensator including a polarization controller and a variable differential polarization delay unit. The polarization controller converts incoming light of an arbitrary polarization to a controlled output signal having a desired state of polarization. The differential polarization delay unit is optically coupled to receive the controller output signal. The differential polarization delay unit includes a polarization beam splitter element, a differential delay element, and a polarization combiner element. The polarization beam splitter has a first input port coupled to receive the controller output signal, a split point, and a first and a second output port, where at the split point the controller output signal is split into a first and a second orthogonal polarization signals. The first and the second polarization signals are directed to the first and second output ports of the splitter respectively. The differential delay element includes a first waveguide and a second waveguide. The waveguides are birefringent, thereby suppressing coupling between the two polarization modes in each. The first waveguide and the first output port of the splitter are optically coupled and aligned by matching their cores and polarization axes. The first waveguide has a first chirped grating tuned to reflect the first polarization signal at a first reference reflection point. The second waveguide is optically coupled and aligned to the second output port of the splitter. The second waveguide has a second chirped grating tuned to reflect the second polarization signal and has a second reference reflection point. The chirp of the gratings may be linear, or may have a more complex spatial dependence.
At least one tuning mechanism is coupled to at least one of the gratings. The tuning mechanism is capable of variably adjusting the optical path length of one or both of the reference points, with respect to the split point. The tuning mechanism may include: applying axial mechanical stress to stretch the gratings, applying electric fields to electro-optically control the grating index, applying heat to thermo-optically control the grating index, or using other tuning mechanisms known in the art. In one embodiment, the tuning mechanism includes a first tuning device and a second tuning device. The first tuning device is coupled to both the first and second gratings and tunes both gratings generally simultaneously and in equal amounts. The second tuning device independently tunes only one of the gratings.
The initial position of the first and second reference reflection points with respect to the split point (i.e., the optical path length of the segment) may be tailored to the particular application. In applications where the expected DGD does not exceed the range of the tuning mechanism, the first and second reference reflection points may be at substantially the same optical path length with respect to the split point. Alternatively, one or the other reference reflection points may be biased, that is, have a different optical path length, to compensate for all or part of the PMD. A combiner element recombines the two reflected orthogonal polarization signals into a delay line output. In a preferred embodiment, a splitter/combiner performs the functions of both the splitter and the combiner elements.
A circulator may be used to route the input and output signals. The circulator has an input port optically coupled to receive the polarization controller output signal, a recirculation port optically coupled to transmit the polarization controller output signal to the differential polarization delay unit and to receive the delay unit output, and an output port optically coupled to transmit the delay unit output.
An optical tap coupler may be coupled to the output port of the circulator to provide a signal analyzer with a sample of the output signal. The analyzer evaluates the quality of the delay line output signal and provides control signals to the polarization controller and the differential polarization delay unit.
Different components of the present invention may be integrated into an integrated optical device, such as a LiNbO3 chip, which contains birefringent waveguides. In one embodiment, the polarization controller and the differential polarization delay unit are integrated onto a single LiNbO3 chip. In another embodiment, the polarization dispersion compensator components from neighboring channels in a WDM system may be integrated onto a single LiNbO3 chip. Obviously, integrated optical devices based on other materials systems could also be used.
The present invention further relates to a method for compensating for polarization mode dispersion of an incoming optical communications signal. The incoming optical communications signal is first passed through a polarization controller, which aligns the states of the signal polarization to the optical axes of a differential polarization delay unit. In the delay unit, the communications signal is split into a first and second orthogonal principal state of polarization at a split point. The first polarization state is directed to a first waveguide having a first chirped grating having a first reference reflection point. The second polarization state is directed to a second waveguide having a second chirped grating having a second reference reflection point. The first and second reflections of the optical communications signal are recombined at the polarization combiner. The output signal may be sampled using a signal analyzer to determine its quality. The state of polarization of the incoming signal and/or the optical path length location of the reflection points may then be variably adjusted to optimize the quality of the output signal.
In exemplary embodiments, the waveguides of the differential polarization delay unit are birefringent single-mode optical fiber, or birefringent waveguides in an integrated optical device. Birefringent single mode fiber includes polarization maintaining fiber, polarizing fiber, shaped birefringent fiber, and photonic band gap optical fiber.
In a particular embodiment, the fibers are polarization-maintaining (PM) or polarizing (PZ) single mode silica-based optical fibers and the gratings are linearly chirped and have substantially similar length and chirp patterns. In a specific embodiment for compensation of PMD in the range of 100 ps (for up to one bit period at 10 Gb/sec data rate) in a single wavelength channel of a dense wavelength division multiplexed (DWDM) telecommunications system, the first and second gratings measure at least five (5) cm long, with an optical chirp rate that may be set to accommodate the level of chromatic dispersion of the incoming signal. A specific exemplary embodiment for polarization delay of 100 ps includes first and second gratings having a length of 5 cm and a chirp rate of 680 ps/nm for a signal of wavelength 1550 nm.
The optical path length location of one or both of the reference reflection points is adjustably varied to compensate for polarization dispersion between the first and second orthogonal states of polarization. In an alternative embodiment, prior to the step of adjustably varying the optical path length from the second reflection point, the optical path length of at least one of the gratings may be pre-tuned such that one of the reflection points is either at substantially the same optical path length as the other reflection point or slightly ahead or behind (shorter or longer optical path length).